A system is known that records and reproduces video and audio information with the aid of an optical information recording medium having a recording layer sensitive to laser light. In this system, information is recorded in the form of a series of pits that are made on the surface of an optical information recording medium such as an optical recording disk by illumination with a laser beam suitably modulated in response to an information signal.
A cross section of the conventional optical information recording medium is shown in FIG. 1. A substrate 1 is made of a material selected from among glass, acrylic resins, vinyl resins, polyether resins, epoxy resins, polycarbonate resins, polybutyral resins, cellulose acetate butyrate resins, and nitrocellulose resins. That substrate 1 is overlaid with a recording layer 2 in the form of a thin film that is made of a material which, when heated by a laser light, sublimes to leave a pit in its surface. An example of the material that sublimes upon scanning with a laser beam is a dye sensitizer and a binder (e.g. nitrocellulose) dissolved in a solvent such as ketone.
A recording layer having a uniform thickness is provided by a spin coating technique. Under this method, while the substrate is rotated about its axis at a speed of preferably from 200 to 250 rpm, a solution of a light-sensitive material for making the recording layer 2 is dripped onto the substrate through a nozzle slowly moving in the radial direction. After the dripping is discontinued, the rotating speed of the substrate 1 is increased to, preferably 400 to 1000 rpm so as to spin off the excess solution and to provide the substrate 1 with a uniform layer of solution that contains the light-sensitive material. Subsequently, the solution layer is dried to evaporate the solvent and form the desired recording layer 2.
It has been found to be difficult to use a single light-sensitive material to obtain a recording layer that exhibits good sensitivity characteristics with a laser light having a certain wavelength. In order to overcome this difficulty, a recording layer made of a mixture of two or more light-sensitive layers has been proposed. However, this idea suffers from the problem of there being a small latitude in the selection of light-sensitive materials because the solution with which the substrate is to be spin coated must be prepared using a solvent that is miscible with all the light-sensitive materials used. Another requirement that must be considered is that the miscible solvent used should not dissolve, swell, or cause otherwise adverse effects on the substrate 1. This consideration puts strict limitations on the usable combinations of light-sensitive materials and solvents.
The use of a thin film containing an organic dye, especially a cyanine dye, as the recording layer on an optical disk is shown in Japanese Published Patent Application Nos. 114989/1983 and 112790/1983, and Japanese Patent Application No. 210077/1983. Optical disks incorporating this idea are shown in FIG. 4.
A substrate 11 formed in a disk shape from acrylic acid resins such as polymethyl methacrylate (PMMA) by the P.P. (Photo-Polymer) method or injection method is provided with fine grooves either in the form of concentric circles or in a helix (the grooves are hereunder referred to as guide tracks).
In the cleaning step, any dirt present is removed from the surface of the substrate 11. A solution of a cyanine dye is spin coated onto the guide tracks on the substrate 11 to form a uniform coating. In the subsequent baking step, the coated surface is dried to form recording layer 12. A protective PMMA plate 14 is bonded to the recording layer 12 on the substrate 11, with a spacer 16 being interposed to form a gap 13. A cross section of the optical disk is shown in FIG. 2. A double-side optical disk using two substrates 11 each having a recording layer 12 as shown in FIG. 3 has also been proposed.
The spin coating method is performed in the above-described manner. While the substrate 11 is rotated about its center at a speed of 200 to 250 rpm, a solution of cyanine dye is dripped onto the substrate through a dispenser slowly moving in the radial direction. After the dripping is stopped, the rotating speed of the substrate 11 is increased to 400 to 1000 rpm so as to spin off the excess solution and to provide the substrate with a uniform layer of the solution.
The recording layer 12 on the conventional optical disk is a thin film of a cyanine dye and has a bronze-like or metallic gloss suitable for reflecting a laser light in the reproduction mode. The thin film of cyanine due sublimes by absorbing a specific wavelength component of the laser light, thus also working as a recording layer wherein pits are formed by illumination with the laser light in the recording mode. The thin film of cyanine dye is flexible and it has been confirmed that unlike metal films, this film is less likely to develop cracks even if temperature changes occur. Because of these advantages, the conventional optical disk uses a thin film made of a single cyanine dye component.
However, most of the solvents capable of dissolving cyanine dyes are ketone compounds which dissolve the PMMA that is extensively used as a substrate material. Stated more specifically, when a solution of a cyanine dye in a ketone solvent is spin coated onto a PMMA substrate with guide tracks, the solution swells or dissolves its surface, and the resulting thin film of cyanine dye does not have the desired metallic gloss. Therefore, a recording layer in the form of a thin film of a single cyanine dye component has not been suitable for use with commercial optical disks.